The present invention relates to a siren or alarm unit, and in particular a siren or alarm unit which can be used with vehicle security systems.
Vehicle security systems may include sirens which are activated by the system when an alarm condition occurs, such as unauthorised or forced entry to a vehicle. The units which drive the siren are normally configured to generate a predetermined siren signal of a set volume and duration. The frequency of activation of the siren may also be set, and in some instances the frequency is varied during the period of activation in accordance with a set frequency or sound pattern.
Rigid setting of the frequency, volume and duration is disadvantageous as regional authorities have established different detailed specifications which place controls of the frequency, volume and duration for use of vehicle sirens. For example, an authority may specify maximum and minimum frequencies which can be used, or limits on the predominate frequency, for any vehicle siren signals generated. This means that different siren units have to be manufactured for each region, country or state depending on the authority's requirements. These requirements or regulations then need to be taken into account to ensure the correct siren unit is installed in a vehicle, and attention paid to the likely region, country or state in which the vehicle will be used. This still, of course, does not overcome the problem of a vehicle owner breaching a local authority's regulations when the vehicle's siren is activated in another region, country or state from that which it is normally driven in.